Belt tensioner

ABSTRACT

A belt tensioner with a retainer can be brought into contact with a belt via a belt support for applying a tensile force. The retainer is joined at a distance from the belt support to a radial plain bearing and it can be swivelled about the axis of rotation of the radial plain bearing, whereby the tensioning and swivel movements of the belt support occur under the action of a coil spring. For a belt tensioner of this type, which with a compact construction runs essentially wear-free, it is suggested that the retainer is subject to the action of a spring, which exerts a force (F 3 ) essentially parallel to the axis of rotation on the retainer, the said force (F 3 ) counteracting the force (F 1 ) exerted by the belt on the retainer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a belt tensioner of the type for automotivevehicles.

2. Description of the Related Art

Typical belt tensioners are known, for example from DE-A-43 27 141 orDE-A-40 10 928. These belt tensioners are employed in motor vehicles fortightening the V-belt and comprise a retainer with a tension arm, whichis fitted with an idler pulley at one end, that is applied to the belt,and which is joined to a radial plain bearing at its other end. Theradial plain bearing includes a bearing bush part and a bearing pinpart, whereby one of the parts is fixed to the tension arm and the otherpart is held spatially fixed. A rotary spring in the form of a spiralcoil spring is wrapped coaxially around the bearing bush and bearing pinand is held by one end on the retainer and held spatially fixed at theother end. These known belt tensioners are fitted in a state in whichthe bearing bush and the bearing pin have been rotated against oneanother under the pretension of the rotary spring, so that the idlerpulley already exerts a predetermined force on the belt. Due to thespring force of the pretensioned rotary spring, the belt is held under apredetermined tension, but the idler pulley can deflect or take up theslack if the length of the belt changes.

Due to the type of construction, the tension arm bearing the belt idlerpulley must, however, be arranged off center in relation to the axiallength of the radial plain bearing on the bearing bush or bearing pin.Consequently, the radial plain bearing is, however, stressed by tiltingforces which cause increased wear. Attempts have been made in DE-A-43 27141 to arrange the deflecting point of the swivel spring in the axialdirection to the rotating axis so far removed from the radial plainbearing that the resultant of the force introduced into the radial plainbearing meets approximately the center of the axial length of the radialplain bearing. This is however only possible where there is sufficientinstallation space available.

The object of the invention is to develop a belt tensioner such that thesusceptibility to wear is reduced with a compact, short construction.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a belt tensioner is providedwith a retainer which can be brought into contact with a belt via a beltsupport for applying a tensile force and which, at a distance from thebelt support, is connected to a radial plain bearing and can beswivelled about its axis of rotation, whereby the tensioning and swivelmovements of the belt support occur under the load of a coil spring. Theinvention is characterized in that the retainer is subject to the actionof a spring, which exerts a force essentially parallel to the axis ofrotation on the retainer and the force counteracting upon the forceexerted by the belt on the retainer.

According to the invention the retainer is subject to the action of aspring which acts against the tilting moment which is caused by theforce exerted through the belt on the retainer. The application of thisspring is possible with a compact construction, so that the installationspace required for fitting does not need to be enlarged.

Although belt tensioners are already known which exhibit more than onespring, such as for example the belt tensioner according to DE-A-26 08277, the springs in this case, however, have a different function. Withthis known belt tensioner, a first compression spring is used whichpresses friction discs against the retainer and whose spring forcedefines a threshold value at which the retainer can only then beswivelled. The belt tensioner also has a second compression spring whichis formed as a spring strut and presses the idler pulley directlyagainst the belt.

The application of springs, which act on the retainer essentiallyparallel to the rotating axis, is for example known with a belttensioning arm according to DE-A-195-24-403. Here however, this springis used as a single spring and in its function replaces the rotaryspring of the generic state of the art, i.e. the tensile force isdetermined through the pretension of the cup spring.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention will be readily appreciated as thesame becomes better understood by reference to the following detaileddescription when considered in connection with the accompanying drawingswherein:

FIG. 1 a partial cross-section through a first embodiment of a belttensioner according to the invention, and

FIG. 2 a partial cross-section through a second embodiment of a belttensioner according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 illustrates a belt tensioner 1 in partial cross-section as it isused, for example, for tensioning belts in automotive vehicles. The belttensioner 1 is however also suitable for other applications where belts,chains or other endless transmission elements need to be tensioned.

The belt tensioner 1 includes a base section 2 which is provided with acentral opening 3 and cup-shaped, high-reaching walls 4 which form partof a housing. The belt tensioner 1 also includes a retainer 5, whichalso exhibits a central opening 6 and cup-shaped high-reaching sidewalls 7, whereby the walls 4 and 7 are formed such that they form theouter boundary of the belt tensioner 1 and the center lines of theopenings 3 and 6 meet in a common center line 8.

The retainer 5 is also provided with a tension arm 9 which is mounted inthe axial direction asymmetrically and is offset from the center line 8on the retainer 5 and protrudes over the wall 7. An idler pulley 10 isrotationally supported about an indicated axis 11 in the tension arm 9.The idler pulley 10 forms a belt support for the belt to be tensioned,whereby the axis 11 runs parallel to the center line 8.

One end of a hollow bearing pin 12 is mounted in the opening 3 of thebase section 2, preferably by caulking. A screw, which is not shown, canbe passed through the hollow bearing pin 12, enabling the belt tensioner1 to be firmly mounted onto an engine part of a motor vehicle orsimilar.

The retainer 5 with a bearing bush 13 sits on this hollow bearing pin12, whereby the bearing bush 13 and bearing pin 12 can be displacedrelative to one another in the axial direction and the inner surface ofthe bearing bush 13 forms a radial plain bearing 14 with the outersurface of the bearing pin 12, so that the retainer 5 can rotate aboutthe bearing pin 12, whereby the center line 8 forms the axis ofrotation. The retainer 5 is held by a locking piece 15 in the form of adisc on the bearing pin 12, the disc forming a positive locking jointwith the free end of the bearing pin 12 opposite the base section 2.

The retainer 5 also includes a friction cone 16 which extends coaxiallyon the center line 8 and on which a spring bush 17 and a coil bush 18are arranged in a normal manner, carrying a rotary spring 19 in the formof a spiral-coil spring. The rotary spring 19 is, as is usual with belttensioners of this type, attached at one end to the base section 2 andto the retainer 5 at its other end, so that tensioning can be providedthrough the relative rotation of the retainer 5 and the base section 2.The function and the operation of tensioning with the aid of this typeof coil spring is known to the specialist so that further details neednot be supplied here.

Through the tensioning of the belt it exerts a force F1 on the retainer5. Since the bearing bush 13 is spaced in the axial direction from theidler pulley 10 with regard to the axis of rotation 8, and since,compared to the belt tensioner 1 of the generic state of the artaccording to DE 43 27 141, the friction cone 16 and therefore the lowermounting point of the coil spring 19 for introducing a counter force F2in the axial direction of the axis of rotation 8 is located closer tothe bearing bush 13 than the idler pulley 10, this would lead to aresultant force, which is applied off center on the plain bearing 14,and would therefore cause a tilting moment, which would lead toincreased wear of the radial plain bearing 14.

To prevent this, the bearing bush 13 of the radial plain bearing 14 hasa shorter axial length than the intervening space between the lockingdisc 15 and the base section 2 in the region around the opening 3. Inthe axial intervening space produced by this, a spring 20 is arrangedwhich is formed as a ring-shaped cup spring on which the bearing pin 12is located and acts symmetrically on the axis of rotation 8. In theregion of this cup spring 20 preferably the base section 2 is fittedwith a base 21 which acts as a thrust pad so that the cup spring 20 isevenly compressed. The cup spring 20 acts via a supporting ring 22 onthe bearing bush 13 such that a force F₃ can be exerted on the bearingbush 13 parallel to the axis of rotation 8, the force F₃ supporting F₂as counter force for the introduced force F₁ transferred from the belt,so that essentially a resulting force FR aligned to the axial center ofthe radial plain bearing 14 is produced with which a tilting moment isessentially not produced.

During assembly, the force F₃ can be set variably via the springdisplacement of the cup spring 20, whereby values between about 4000 to7000 N are preferred.

The cup springs 20 act via the supporting ring 22 on a face of thebearing bush 13 and press it against the locking disc 15 which acts asan abutment. Here, the sides facing one another are formed as axialplain bearings between the locking disc 15 and the bearing bush 13 orbetween the bearing bush 13 and the supporting ring 22. These axialplain bearings preferably have steel surfaces which are coated withPTFE. Depending on the type of coating, these plain bearings can alsohowever contribute to friction damping, so that the applied damping issplit between the damping due to the spring 19 and the damping due tofriction.

The plain bearings can also be provided by separate discs and/or inanother axial position.

In an illustration similar to FIG. 1, FIG. 2 shows a second embodimentof a belt tensioner 30, whereby components similar to the firstembodiment are identified with the same reference symbols and are notexplained again.

The belt tensioner 30 differs from the belt tensioner 1 only due to thefact that here an additional, increased friction damping is produced bythe arrangement of a special damping washer 31. The damping washer 31 ispreferably placed between the bearing bush 13 and the locking disc 15,whereby, due to the cup spring 20, the bearing bush 13 is pressed viathe supporting ring 22 against the damping washer 31 which it pressesagainst the locking disc 15. This arrangement is particularlyadvantageous where additional friction damping is required due to veryhigh application-related requirements. Here, friction damping values ofup to 60% of the tensile force or the torque can be produced. Wear-freefunctioning can be ensured due to tuning the damping via the coil spring19 on one hand and via the spring 20 on the other, or due to splittingof the damping between both systems, despite an overall damping value of85% referred to the torque.

An advantageous material pairing for the damping washer 31 and theadjacent locking disc 15 is for example a glass-fibre reinforcedpolyamide, in particular PA 46 with 5% glass-fibre content, for thedamping washer 31 and a stainless steel, in particular V2A, for thelocking disc 15. The plain bearings, which are arranged in this casebetween the bearing bush 13 and the damping washer 31 or between thebearing bush 13 and the supporting ring 22, can, as with the firstembodiment, have bearing surfaces of steel coated with PTFE, which,where required, are arranged on additional washers not illustrated inthe drawings.

In a modification of the described and drawn embodiments a differentsequence of the components is possible. For example, the cup spring canbe arranged on the side of the bearing bush facing the locking disc.Instead of a cup spring a different suitable spring can be used. Theselection of materials can be carried out with regard to the servicelife to be achieved and/or to the required damping values. The inventioncan furthermore be applied when the tension arm is joined to the bearingpins.

1. A belt tensioner with a retainer, the retainer adapted to be broughtinto contact with a belt via a belt support for applying a tensile forceand which, at a distance from the belt support, is connected to a radialplain bearing and adapted to be swivelled about its axis of rotation,whereby the tensioning and swivel movements of the belt support occurunder the load of a coil spring, characterized in that the retainer issubject to the action of a spring, which exerts a force (F₃) essentiallyparallel to the axis of rotation on the retainer, the force (F₃)counteracting the force (F₁) exerted by the belt on the retainer.
 2. Abelt tensioner according to claim 1, wherin the radial plain bearingincludes a bearing bush and a bearing pin, which can be displacedrelative to one another in the axial direction of the axis of rotationagainst the force of the spring.
 3. A belt tensioner according to claim2, wherein the retainer is provided with the bearing bush and the springacts on the bearing bush.
 4. A belt tensioner according to claim 3,wherein the spring is a cup spring which is arranged coaxially about theaxis of rotation.
 5. A belt tensioner according to claim 4, wherein asupporting ring is arranged between the cup spring and the bearing bush,whereby an axial plain bearing is formed between the supporting ring andthe bearing bush.
 6. A belt tensioner according to claim 5, wherein theforce (F₃) exerted by the spring is taken up by an abutment having anaxial plain bearing and a supporting piece.
 7. A belt tensioneraccording to claim 6, wherein a damping device for damping the swivelmovements of the retainer about the axis of rotation is subject to theaction of the further spring.
 8. A belt tensioner according to claim 7,wherein the damping device includes a damping washer of a glass-fibrereinforced plastic, in particular a polyamide with about 5% glass-fibrecontent and a locking disc of steel, in particular of stainless steel,adjacent to the damping washer.
 9. A belt tensioner according claim 6,wherein the radial plain bearing includes a bearing pin on which arearranged in the axial direction with regard to the axis of rotation inthe following sequence: a locking piece; a cup spring; a bearing bushjoined to the retainer; and a locking piece.
 10. A belt tensioneraccording to claim 6, wherein the radial plain bearing includes abearing pin on which are arranged in the axial direction with regard tothe axis of rotation in the following sequence: a locking piece; a cupspring; a bearing bush joined to the retainer; a damping washer; and alocking piece.